This invention describes a magnetic read-write head used in disk drives.
The technological advance of the digital recording and storage of information has produced a concomitantly rising need for greater capacity, speed and precision in the disk drive equipment used for this purpose.
For a review of the magnetic recording technology, the reader is directed to the following references, among others: Robert M. White, Editor, "Introduction to Magnetic Recording:, IEEE Press, New York, 1985, P. 51; W. A. Gross, "Fluid Film Lubrication", Wiley Interscience, New York, 1980; and C. Dennie Mee and E. D. Daniel, Editors, "Magnetic Recording, Vol. I - Technology", McGraw-Hill, New York, 1987, Chapter 7.
The critical component in a digital recording system is represented by the recording head which is usually held in a spring-loaded holder and placed onto the disk surface.
During the operation of a disk drive, as the disk rotates, the recording head is airborne and maintains a small spacing between itself and the disk surface. In the disk drive art, this is referred to as the head "flying" above the disk. The spacing is in the order of 6-12 microinches (0.15-0.3 micrometers). The recording head is assembled onto a spring and is loaded against the disk surface with a force provided by the spring. As the disk rotates, a self-acting hydrodynamic air bearing or cushion is formed between the air bearing surfaces and the disk surface. This air bearing provides the pressure against the force of the spring to maintain a constant separation between the head and the disk.
As is well known in the art of rigid disk recording technology, the flying height of the head above the disk is a function of many variables. Two are among the most important, namely, width of the air bearing surfaces and the relative velocity between the air bearing surfaces.
At present, there are three widely used, basic kinds of recording heads in the field of magnetic digital recording. These are ferrite monolithic heads, composite heads and thin film heads. Both the monolithic and composite heads utilize ferrite materials in their magnetic read-write structures and are therefore the main concern of this disclosure.
In the monolithic head, as the name implies, the read-write transducer as well as the body of the device, are formed together from the same piece of ferrite. The composite head, on the other hand, is a composite structure which consists of a transducer made from ferrite and a body made from a non-magnetic, usually ceramic material. The ferrite transducer and the ceramic body are manufactured separately and then are assembled and bonded together to form a composite structure.
Both heads have their distinct advantages and disadvantages when compared to each other. The greatest advantage of the monolithic recording head is its obvious ease of mass manufacturability with a resulting lower cost. The composite head, on the other hand, which involves assembling and bonding of at least two separate parts, is more difficult and costly to manufacture. The construction of composite heads requires machining, grinding, lapping and handling of very small and delicate ferrite cores. The small core portion for a composite head has to be precisely positioned in relation to the ceramic body and bonded to it by glass flow. Handling delicate and fragile parts and precisely positioning same for head assembly can be an expensive process.
The bonding of the two parts is usually done by glass bonding. Glass bonding technology is widely accepted and used in recording head technology as a clean and dependable method for permanently and precisely bonding two parts together. However, melting glass requires high temperatures. In the case of composite heads, this glass bonding operation is also the source of some further disadvantages. The operation, e.g., requires precision jigs which must endure the high glass bonding temperatures.
Moreover, the ferrite core must be embedded and bonded to the non-ferrite base such that the resulting head structure will be free of stresses and remain physically durable. As is well known in the field, magnetic materials such as ferrite, do suffer degradation of their magnetic properties when put under stress from temperature and other changes. For example, saturation magnetization of the ferrite core can be degraded by stress through a phenomenon known as magnetostriction. Therefore, the glass chosen for bonding the composite heads must have thermal expansion and contraction coefficients closely matched to that of the ferrite core and the ceramic body. The thermal expansion and contraction properties of the ceramic slider body must also closely match that of the ferrite core. These constraints seriously limit the choice of glasses for bonding ferrite cores into ceramic bodies to make composite sliders. Glass which may be suitable from a thermal expansion point of view is often found to be lacking in chemical and environmental stability. But the monolithic heads, not needing a bonding of the type described, are free of such problems.
There is a constant requirement to increase the density of stored information on a disk surface. This is accomplished by increasing the number of circular tracks on the disk surface and the bits stored in each track. This is commonly referred to as increasing the TPI, tracks per inch along a radius, and increasing the BPI, bits per inch of track, respectively. The requirement to increase the TPI is satisfied by making the read-write portion of the magnetic transducer, commonly referred to as track width, narrower.
Composite heads have the advantage over monolithic heads in this area because they can be made with durable narrow track widths. The machining of the composite ferrite cores with a narrow track width is not an easy task, but once they are securely embedded in glass, they remain protected and durable in use. Therefore, composite heads provide the disk drive industry with the desirable narrow track capability. However, the means by which the composite heads with narrow tracks are achieved are costly and require dissimilar materials.
The monolithic heads can also be made with narrow tracks but the resulting structure has an exposed, unprotected tracking ridge extending the entire length of the slider body. This narrow section of the head, including the read-write gap, is vulnerable to physical damage. Chipping of the narrow track edges results in a diminished read-write performance. The chipped ridge also gives unpredictable "flying" characteristics by changing the airflow pattern. It can be concluded, therefore, that the monolithic recording heads presently do not provide very narrow track widths together with reliability and durability.
For high density digital disk storage, another advantage of composite heads is obtained by placing the transducer at the end of an air bearing surface rather than at the middle of the trailing edge of the slider. This feature enables the device to record still more information on a given disk surface. This important advantage cannot be obtained from monolithic heads as presently known.